You need to think of things like that like a low-scale industrial system. It needs to do what it does.
If you go to a candy-store at the beach and see a 150 year old saltwater taffy machine, you say "ooh, cool". The thing that makes a missile go up is essentially the same thing, it needs perform it's function to spec, period.
Yup. In tons of industrial settings it's common to have machines that are a century or more old.
I know someone who works for an industrial gear company and routinely reconditions things like draw bridge mechanisms or a rubber rolling mill (do they call it a mill?) and most customers just want things to keep on working like they've always been working since they've built their process around them and machine throughput is rarely the bottle neck. The improvements customers go for are modern bearings (quiet and cheaply serviceable) and if they have to have gears made they generally opt for a more modern tooth profile (potentially stronger, quieter and more efficient).
If you go to a candy-store at the beach and see a 150 year old saltwater taffy machine, you say "ooh, cool". The thing that makes a missile go up is essentially the same thing, it needs perform it's function to spec, period.